Fix warnings about the `native` target-cpu

This fixes a regression from #53031 where specifying `-C target-cpu=native` is
printing a lot of warnings from LLVM about `native` being an unknown CPU. It
turns out that `native` is indeed an unknown CPU and we have to perform a
mapping to an actual CPU name, but this mapping is only performed in one
location rather than all locations we inform LLVM about the target CPU.

This commit centralizes the mapping of `native` to LLVM's value of the native
CPU, ensuring that all locations we inform LLVM about the `target-cpu` it's
never `native`.

Closes #53322

8 files changed, 49 insertions, 18 deletions

diff --git a/src/librustc/session/mod.rs b/src/librustc/session/mod.rs
index ef81cd3a457..c8c0d4c38a2 100644
--- a/src/librustc/session/mod.rs
+++ b/src/librustc/session/mod.rs
@@ -649,13 +649,6 @@ impl Session {
         }
     }
 
-    pub fn target_cpu(&self) -> &str {
-        match self.opts.cg.target_cpu {
-            Some(ref s) => &**s,
-            None => &*self.target.target.options.cpu
-        }
-    }
-
     pub fn must_not_eliminate_frame_pointers(&self) -> bool {
         if let Some(x) = self.opts.cg.force_frame_pointers {
             x
diff --git a/src/librustc_codegen_llvm/attributes.rs b/src/librustc_codegen_llvm/attributes.rs
index 2b64642b766..c8a2b47e98d 100644
--- a/src/librustc_codegen_llvm/attributes.rs
+++ b/src/librustc_codegen_llvm/attributes.rs
@@ -124,7 +124,8 @@ pub fn llvm_target_features(sess: &Session) -> impl Iterator<Item = &str> {
 }
 
 pub fn apply_target_cpu_attr(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
-    let target_cpu = CString::new(cx.tcx.sess.target_cpu().to_string()).unwrap();
+    let cpu = llvm_util::target_cpu(cx.tcx.sess);
+    let target_cpu = CString::new(cpu).unwrap();
     llvm::AddFunctionAttrStringValue(
             llfn,
             llvm::AttributePlace::Function,
diff --git a/src/librustc_codegen_llvm/back/linker.rs b/src/librustc_codegen_llvm/back/linker.rs
index a429e8f2d81..eee60b262a1 100644
--- a/src/librustc_codegen_llvm/back/linker.rs
+++ b/src/librustc_codegen_llvm/back/linker.rs
@@ -26,6 +26,7 @@ use rustc::session::config::{self, CrateType, OptLevel, DebugInfo,
 use rustc::ty::TyCtxt;
 use rustc_target::spec::{LinkerFlavor, LldFlavor};
 use serialize::{json, Encoder};
+use llvm_util;
 
 /// For all the linkers we support, and information they might
 /// need out of the shared crate context before we get rid of it.
@@ -202,7 +203,7 @@ impl<'a> GccLinker<'a> {
         };
 
         self.linker_arg(&format!("-plugin-opt={}", opt_level));
-        self.linker_arg(&format!("-plugin-opt=mcpu={}", self.sess.target_cpu()));
+        self.linker_arg(&format!("-plugin-opt=mcpu={}", llvm_util::target_cpu(self.sess)));
 
         match self.sess.opts.cg.lto {
             config::Lto::Thin |
diff --git a/src/librustc_codegen_llvm/back/write.rs b/src/librustc_codegen_llvm/back/write.rs
index 80e9965e39c..2373428d68c 100644
--- a/src/librustc_codegen_llvm/back/write.rs
+++ b/src/librustc_codegen_llvm/back/write.rs
@@ -25,6 +25,7 @@ use rustc::session::Session;
 use rustc::util::nodemap::FxHashMap;
 use time_graph::{self, TimeGraph, Timeline};
 use llvm::{self, DiagnosticInfo, PassManager, SMDiagnostic};
+use llvm_util;
 use {CodegenResults, ModuleSource, ModuleCodegen, CompiledModule, ModuleKind};
 use CrateInfo;
 use rustc::hir::def_id::{CrateNum, LOCAL_CRATE};
@@ -173,7 +174,7 @@ pub fn target_machine_factory(sess: &Session, find_features: bool)
     let singlethread = sess.target.target.options.singlethread;
 
     let triple = SmallCStr::new(&sess.target.target.llvm_target);
-    let cpu = SmallCStr::new(sess.target_cpu());
+    let cpu = SmallCStr::new(llvm_util::target_cpu(sess));
     let features = attributes::llvm_target_features(sess)
         .collect::<Vec<_>>()
         .join(",");
diff --git a/src/librustc_codegen_llvm/llvm/ffi.rs b/src/librustc_codegen_llvm/llvm/ffi.rs
index d3039a05b6d..51b0299e63f 100644
--- a/src/librustc_codegen_llvm/llvm/ffi.rs
+++ b/src/librustc_codegen_llvm/llvm/ffi.rs
@@ -1449,6 +1449,7 @@ extern "C" {
     pub fn LLVMRustPrintTargetCPUs(T: &TargetMachine);
     pub fn LLVMRustPrintTargetFeatures(T: &TargetMachine);
 
+    pub fn LLVMRustGetHostCPUName(len: *mut usize) -> *const c_char;
     pub fn LLVMRustCreateTargetMachine(Triple: *const c_char,
                                        CPU: *const c_char,
                                        Features: *const c_char,
diff --git a/src/librustc_codegen_llvm/llvm_util.rs b/src/librustc_codegen_llvm/llvm_util.rs
index ff26e0f35f0..9fcc33d82cf 100644
--- a/src/librustc_codegen_llvm/llvm_util.rs
+++ b/src/librustc_codegen_llvm/llvm_util.rs
@@ -17,6 +17,8 @@ use libc::c_int;
 use std::ffi::CString;
 use syntax::feature_gate::UnstableFeatures;
 
+use std::str;
+use std::slice;
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::Once;
 
@@ -262,3 +264,19 @@ pub(crate) fn print(req: PrintRequest, sess: &Session) {
         }
     }
 }
+
+pub fn target_cpu(sess: &Session) -> &str {
+    let name = match sess.opts.cg.target_cpu {
+        Some(ref s) => &**s,
+        None => &*sess.target.target.options.cpu
+    };
+    if name != "native" {
+        return name
+    }
+
+    unsafe {
+        let mut len = 0;
+        let ptr = llvm::LLVMRustGetHostCPUName(&mut len);
+        str::from_utf8(slice::from_raw_parts(ptr as *const u8, len)).unwrap()
+    }
+}
diff --git a/src/rustllvm/PassWrapper.cpp b/src/rustllvm/PassWrapper.cpp
index 46825efeee2..09befdaae37 100644
--- a/src/rustllvm/PassWrapper.cpp
+++ b/src/rustllvm/PassWrapper.cpp
@@ -359,6 +359,12 @@ extern "C" void LLVMRustPrintTargetFeatures(LLVMTargetMachineRef) {
 }
 #endif
 
+extern "C" const char* LLVMRustGetHostCPUName(size_t *len) {
+  StringRef Name = sys::getHostCPUName();
+  *len = Name.size();
+  return Name.data();
+}
+
 extern "C" LLVMTargetMachineRef LLVMRustCreateTargetMachine(
     const char *TripleStr, const char *CPU, const char *Feature,
     LLVMRustCodeModel RustCM, LLVMRustRelocMode RustReloc,
@@ -381,11 +387,6 @@ extern "C" LLVMTargetMachineRef LLVMRustCreateTargetMachine(
     return nullptr;
   }
 
-  StringRef RealCPU = CPU;
-  if (RealCPU == "native") {
-    RealCPU = sys::getHostCPUName();
-  }
-
   TargetOptions Options;
 
   Options.FloatABIType = FloatABI::Default;
@@ -417,7 +418,7 @@ extern "C" LLVMTargetMachineRef LLVMRustCreateTargetMachine(
   if (RustCM != LLVMRustCodeModel::None)
     CM = fromRust(RustCM);
   TargetMachine *TM = TheTarget->createTargetMachine(
-      Trip.getTriple(), RealCPU, Feature, Options, RM, CM, OptLevel);
+      Trip.getTriple(), CPU, Feature, Options, RM, CM, OptLevel);
   return wrap(TM);
 }
 
diff --git a/src/test/run-make-fulldeps/target-cpu-native/Makefile b/src/test/run-make-fulldeps/target-cpu-native/Makefile
index 0c9d93ecb2a..fee41461612 100644
--- a/src/test/run-make-fulldeps/target-cpu-native/Makefile
+++ b/src/test/run-make-fulldeps/target-cpu-native/Makefile
@@ -1,5 +1,20 @@
 -include ../tools.mk
 
-all:
-	$(RUSTC) foo.rs -C target-cpu=native
+# I *really* don't want to deal with a cross-platform way to compare file sizes,
+# tests in `make` sort of are awful
+ifeq ($(TARGET),x86_64-unknown-linux-gnu)
+all: $(TMPDIR)/out.log
+	# Make sure no warnings about "unknown CPU `native`" were emitted
+	if [ "$$(wc -c $(TMPDIR)/out.log | cut -d' ' -f 1)" = "0" ]; then \
+	  echo no warnings generated; \
+	else \
+	  exit 1; \
+	fi
+else
+all: $(TMPDIR)/out.log
+endif
+
+
+$(TMPDIR)/out.log:
+	$(RUSTC) foo.rs -C target-cpu=native 2>&1 | tee $(TMPDIR)/out.log
 	$(call RUN,foo)